The present invention relates to a semiconductor heterostructure radiation detector having two spectral sensitivity ranges provided by means of adjacent semiconductor layer regions in which photons having different energies can be absorbed. The photons optically excite charge carriers present in the semiconductor layer regions in such a manner that a photo current can be generated in response to an external electric voltage applied via electrodes provided at the semiconductor layer structure.
In the field of semiconductor radiation detectors, photodiodes are known with conventional p-i-n junctions as well as so-called quantum well intersubband photodetectors (QWIP), whose spectral sensitivity properties can be set according to the selection of material layer systems, layer thickness parameters and the selection of n-doping or p-doping. Conventional photodiodes possess spectral sensitivity in the visible to the near-infrared spectral range. Depending on the selection of material, they can also detect wavelengths in the .mu.m range. So-called quantum well intersubband photodetectors actually have spectral sensitivity ranges in the long wave spectral range, preferably in the range between the 3 and 20 .mu.m range, which can be set by the choice of material and layer thickness parameters.
In addition to performance enhancement and optimization of individual radiation detectors, combinations of radiation detectors, with which electromagnetic radiation of different wavelengths can be detected, are also being examined. For example, so-called two-color detectors are employed in thermography and for the optical discrimination of certain objects within the field detected by the radiation detector.
The article by A. Kock et al.: "Double Wavelength Selective GaAs/AlGaAs Infrared Detector Device", Appl. Phys. Lett. 60, 2011 (1992) proposes combining two QWIP semiconductor structures having different detection wavelengths. The 2-step QWIP system introduced in this article comprises alternating sequences of GaAs/AlGaAs layers.
Quantum well structures differing in the dimensions of the barrier height or the band gap, as well as well-width or layer thickness of the quantum well structure, are employed for setting different spectral sensitivity. The QWIP structures conditioned for detecting different wavelengths, however, are separated by an additional doped contact layer. Although the physical separation attained in this manner has the advantage that both QWIP structures can be separately optimized to their respective operating wavelength, this arrangement has the drawback that, due to the separation, at least one additional electrode is required for voltage supply.
Therefore, for rationalization purposes, an attempt has been made to operate the detector structure described in the aforementioned publications with a not connected, additional electrode. (See K. L. Tsai et al. "Two-Color Infrared Photodetector Using GaAs/AlGaAs and Strained InGaAs/AlGaAs Multiquantum Wells", Appl. Phys. Letter 62, 3704, (1993).) Operation of detector structures of this type has revealed that the relative sensitivity for the two operating wavelengths can be tuned by applying a suitable voltage. However, there is the disadvantage that the individual series-connected detectors influence each other electrically. Depending on the application of external voltage, the photosensitivity of one of the two combined detectors can be raised, with the sensitivity of the other detector being lowered. The overall noise behavior of this detector combination is also determined by the respective detector element not participating in photodetection. Consequently the signal-to-noise ratio of this detection structure is relatively poor.
Furthermore, a two-color detector based on a single QWIP structure having two possible intersubband transitions with wavelengths of 5 .mu.m and 10 .mu.m is suggested by K. Kheng et al.: "Two-color GaAs/(AlGa)As Quantum Well Infrared Detector With Voltage-tunable Spectral Sensitivity at 3-5 and 8-12 .mu.m", Appl. Phys. Letter 61, 666(1992). The selection of operating wavelengths is made possible by the fact that the 5 .mu.m transitions demonstrates photovoltaic and the 10 .mu.m transitions photoconductive behavior. In this case as well, the principal disadvantage is that the noise behavior is also determined at short operating wavelengths by the noise associated with long wavelength detection.
Finally, electrically tunable two-color detectors formed by combining two back-to-back p-i-n photodiodes are known. (See M. P. Reine et al.: "Independently Accessed Back-To-Back HgCdTe Photodiodes: A New Dual-Band Infrared Detector", J. Electronic Mater. 24, 669 (1995).)
The object of the present invention is to provide a semiconductor heterostructure radiation detector which overcomes the deficiencies described above.
This and other objects and advantages are achieved by the radiation detector according to the invention, which has two adjacent semiconductor layer regions sensitive in different spectral ranges which can absorb photons having different energies. These photons optically excite the charge carriers present in the semiconductor layer regions in such a manner that a photo current in the respective semiconductor layer region can be generated in response to an external electric voltage applied via electrodes provided at the semiconductor heterostructure, and spectral sensitivity ranges of both semiconductor layer structures can be set separately without lastingly influencing the overall noise behavior of the two-color detector. In particular, the noise behavior of the two-color detector should be dominated by the noise of the active individual detector. Moreover, it should be possible to set and optimize the spectral sensitivity ranges of both semiconductor layer detectors largely independently of each other.
In the radiation detector according to the present invention, the two adjacent semiconductor layer regions differing in their spectral sensitivity ranges are provided by a combination of a photodiode and a quantum well intersubband photodetector. By combining a photodiode with a QWIP structure according to the present invention, in contrast to previous attempts at realizing electrically tunable two-color detectors, the noise behavior of the invented two-color detector can be determined by the noise of the respective active individual detector.
In a preferred embodiment of the invention, the individual detectors of different construction are applied onto a base substrate. The layer sequence of a p-i-n photodiode is precipitated epitaxially, preferably by molecular beam epitaxy, on top of which the layer sequence of a quantum well intersubband photodetector is applied in immediate succession. Moreover, at least two electrodes are provided, one of which is contacted with the photodiode contact layer opposite the QWIP structure and the other electrode with the top covering layer of the QWIP structure.
Upon application of an external electric voltage to the electrodes so that the p-i-n photodiode is operated in forward direction, the spectral sensitivity of the invented two-color detector is determined by the semiconductor layer region of the QWIP structure. This is so because the photodiode, which is operated in forward direction, possesses a negligible differential intrinsic resistance. Consequently, it does not lastingly influence the sensitivity of the active QWIP structure.
On the other hand, if the external voltage is applied in such a manner that the photodiode is operated in the reverse direction, the sensitivity of the entire two-color detector is determined solely by the photodiode, because the photodiode possesses a high dark resistance, so that the differential intrinsic resistance of the QWIP structure can be neglected.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.